Homework copying was detected in the online homework tutor MasteringPhysics.com. Copying increased as each weekly deadline approached, for problems later in each assignment, and dramatically over the semester. The majority of students copied less than 10% of their problems and worked steadily over the three days before the deadline, whereas repetitive copiers (>30% of problems) exerted little effort early. Importantly, copying homework problems that require analytic answers correlates with a 2.4 standard deviation decline for similar problems on exams but did not significantly correlate with gain on the Mechanics Baseline Test. Repetitive copiers initially had comparable ability in math in physics to non-copiers. Changes in course format and instructional practices that previous self-reported academic dishonesty surveys and the observed copying patterns suggested would reduce copying have been accompanied by more than a factor of four reduction of copying from about 11% of all electronic homework problems to less than 3%.